Most conventional ion thrusters do not employ any type of shield for the thruster beam. It should be noted here that the ground screen which surrounds the thruster except for the grids is not considered to be a beam shield as the term is used herein. With no beam shield employed, the angular distribution of beam current density from, for example, a mercury ion thruster tends to exhibit a roughly exponential falloff with increasing angles of divergence from about 30.degree. to about 80.degree. and a relatively flat angular divergence from about 80.degree. to as large as 120.degree.. The flat distribution is known to consist nearly exclusively of low energy charge exchange-produced ions. Accompanying the ion flux at all divergence angles, and perhaps outweighing it at the larger angles, is a flux of sputter products and other neutral species. Hence, in ion thruster applications for spacecraft, any spacecraft structure or component in at least the forward hemisphere about the thruster axis is subjected to a flux of ions and neutral molecules, of various energies and elemental composition, whenever the ion thruster is operated.
Many spacecraft structures, components, and instruments are particularly sensitive to, and may be very readily damaged by, such a flux of ions and neutral molecules, particularly when continued over a substantial period of time. Further, exposed electrical insulators may collect enough neutral species and sputter products to develop excessive electrical leakage or even short out. The surface layers or surface protection of exposed solar cells may be sputter eroded by energetic beam ions to such a degree, or coated by the neutral species and sputter products to such an extent, that the solar cells lose significant power generating efficiency. Exposed optical windows of sensitive optical devices, such as star trackers, also may either be coated or sputter eroded sufficiently to affect the performance of the devices. The same may also occur to exposed thermal control surfaces of the spacecraft. Also the ion (and electron) flux, which is actually a dilute plasma, may be sufficiently intense in the region or view of sensitive electromagnetic devices, instruments, or detectors mounted near the ion thruster to directly interfere with their operation.
Prior art ion thruster spacecraft designs have generally sought to overcome the above problems by locating and directing the ion thrusters relative to the structures, components, and instruments of the spececraft most sensitive to the divergent thruster beam so that the interaction is minimized or reduced to tolerable levels. However, since objectionable interaction may occur within a hemispherical solid angle about the thruster beam axis, this design approach can easily impose inefficient or unacceptable constraints on spacecraft design.
The only devices used with ion thrusters to provide some divergent beam shielding have been simple conical beam shields that diverge in the downstream direction of the ion beam. This type of device suffers at least one serious disadvantage, viz, that the device does not provide for reduction or control of the sputter products and degraded beam ions produced upon ion beam impingement of the shield.
Other devices of possible interest include those disclosed in U.S. Pat. No. 3,130,542 (Kuhrts) and U.S. Pat. No. 3,535,880 (Work et al.). The Kuhrts patent discloses a collector system for neutral particles wherein a conical collector which surrounds the discharge beam of an ion engine collects and recirculates un-ionized particles. The Work et al. patent discloses an ion beam deflection system which is primarily used for deflection steering.